I've been tinkering with this API since it was in private beta. Lots of cool data here, but to get anything out of it for a specific geography you generally have to feed it the right FIPS code going in. So the plugin would have to be able do that translation; no big deal for states, but once you start talking counties or cities you have a massive increase in numbers and a lot of disambiguation to deal with.

Though you can also run queries against every instance of some particular geography within another area — e.g., "give me ACS table B02001_002E for every county in Alabama". So the plugin could provide higher-level summaries or rankings. Any ideas?